The effects of a single immunization of melanoma patients with BCG or C. parvum on the blood counts, serum immunoglobulin levels and lymphoid subpopulations were followed by multiple assays over 28 days. C. parvum produced a decrease in the white cell count, lymphocyte count and lymphoid T and sIg+ cell numbers, which recovered within 1 week; BCG did not produce such a marked depression. Both agents were associated with increases in T cell numbers and lymphocyte PHA blastogenesis after the first week; these declined to pre-immunization values by 3-4 weeks. The sIg-bearing cell subpopulation also increased after BCG. Different methods of expression the results were compared and the difficulties of immunological monitoring are discussed.